This invention relates generally to gas insulated systems and more particularly to a method for testing a gas insulated system for the presence of conducting particles therein.
Compressed electronegative and other gases and mixtures are becoming an increasingly common medium for insulating high voltage equipment of many different kinds. One application is in compressed-gas filled coaxial cables for power transmission and gas-insulated compact substations. Gas-filled capacitance standards for power factor measurement, insulation in air-borne equipment, insulation in high-voltage circuit breakers, and gas-insulated power transformers are among other applications. The advantage of using gas in these applications varies according to the application. For example, in transformers the substitution of gas for oil avoids a fire hazard, while in substations and transmission lines the substitution of gas in place of air results in much more compact equipment.
A well-known problem in compressed gas insulated equipment is associated with the presence of free conducting or semiconducting particles which can initiate flashovers in the gas. While in theory it would be possible to assemble equipment in a clean room, and systems commonly include particle trapping means or other methods of neutralizing the effects of particles, there is still a possibility of the introduction of particles during assembly or, for example, their generation by the movement of sliding joints due to thermal expansion during operation. Therefore, gas-insulated systems have a high probability of containing harmful particles, and field testing by the application of voltage after assembly on site is an important part of the acceptance testing procedure.
Testing of large systems with power frequency voltage requires substantial reactive power, and this power requirement increases with the square of the testing voltage. For example, the test at 200 kV rms, 60 Hz, on a cable having a capacitance of 20 microfarads/foot requires approximately 300 volt amperes/foot or over 1.6 megavolt amperes/mile. Power supplies able to deliver reactive powers of this magnitude are large, expensive, and not easily transportable. A typical cost for a resonant test set may be in the $100,000 to $200,000 range. There is therefore a great incentive to find methods of testing which require less reactive power.
Direct current, or DC, testing enables a peak voltage equal to the peak working 60 Hz voltage to be applied to the system with virtually zero reactive power and very low real power requirements. The test equipment is comparatively compact and easily transported. It is the standard method of testing non-gas-insulated high-voltage cables, and some gas-insulated power transmission systems have also been tested in this way. However, tests have shown that the conducting particles which lead to breakdown move much more readily and at higher speeds with DC compared with alternating current, or AC, voltages. In addition to differences in the movement and breakdown behavior, DC may cause particles to move to, and adhere to, insulators where they may be more harmful when AC or impulse voltages are later applied than they would have been if the particles had been left in their original positions. Unless specially designed, particle traps are also less likely to be effective with DC than AC, so that although the particles are readily moved by the DC, there is a lesser probability that they will be trapped. For these reasons, DC testing is not in favor at the present time.
Other methods which have been suggested include utilizing surge generators or low frequency AC, air or nitrogen in place of the commonly used sulfur hexafluoride (SF.sub.6) or utilizing SF.sub.6 at a lower pressure than at normal operating conditions. One disadvantage of the use of lower pressures is that particles shows a tendency to weld or stick perpendicular to the electrode, and may subsequently cause breakdown. Thus, a high gas pressure equal to or above the normal working pressure is to be preferred.